This invention relates generally to voltage measuring and indicating devices, and in particular to a voltage indicator using serial comparison voltage measurement.
In the past it has been popular to employ a parallel string of comparators, each connected to a different reference voltage for comparison with an input voltage signal, for providing voltage measurements. The parallel comparator technique has been employed in analog-to-BCD converters, analog-to-Gray Scale converters, and analog bar graph voltage indicators.
Such a prior art parallel comparator circuit is shown in FIG. 1. A voltage input signal V.sub.IN is applied via an input terminal 10 simultaneously to the inverting (-) inputs of comparators 12-1, 12-2, 12-3, and 12-4. The non-inverting (+) inputs of comparators 12-1 through 12-4 are connected to reference voltages V.sub.1, V.sub.2, V.sub.3, and V.sub.4, respectively, which are provided in an increasingly positive direction in this example by a voltage divider string comprising resistors 14-1, 14-2, 14-3, 14-4, and 14-5 connected in series between a reference voltage supply V.sub.REF and ground. The outputs of comparators 12-1 through 12-4 are coupled in this example to the cathodes of light-emitting diodes (LEDs) 16-1 through 16-4, respectively, the anodes of which in turn are coupled to a voltage source V.sub.CC through respective load resistors 18-1 through 18-4.
When the voltage applied to the - input of any of the comparators 12-1 through 12-4 exceeds the reference voltage applied to the + input, the output of the comparator switches to a low state, turning on the LED in its output circuit. For example, suppose the voltage input signal V.sub.IN applied to the - input of comparator 12-1 exceeds the value of V.sub.1 applied to the + input thereof. The output of comparator 12-1 switches to a low state, turning on LED 16-1. Continuing with this example, suppose the voltage input signal V.sub.IN rises to a value between V.sub.3 and V.sub.4. Comparators 12-2 and 12-3 switch, turning on LEDs 16-2 and 16-3. So now LEDs 16-1, 16-2, and 16-3 are conducting and emitting light, providing an indication that the voltage input signal has a value greater than V.sub.3 but less than V.sub.4.
A major drawback to the parallel comparator technique is the large numbers of comparators that are required to produce the desired result. Associated with the large number comparators are offset and drift problems within the comparators themselves, and attendant power consumption.
It would be desirable to provide a comparison method having the same result as the parallel comparator technique, but without the attendant problems associated with large numbers of comparators which take up room, consume large amounts of power, and exhibit offset and drift that need to be corrected separately.
One way to reduce the number of comparators is to employ an absolute-value amplifier as input circuit, which would permit the use of a single-polarity voltage in determining the output magnitude. However, offset and drift problems associated with such absolute-value amplifiers creates an ambiguity in the level of the output magnitude.